Methods for preparing polyamide-12 are known in the art, wherein polymerization of .omega.-dodecalactam is effected at a temperature within the range of from 180.degree. to 330.degree. C. in the presence of different catalysts and molecular-mass regulators (cf. French Pat. No. 1,401,842 Cl. C08 g, 1965; French Pat. No. 2,262,680, Cl. C 08 G, 1975; French Pat. No. 1,495,816 Cl. C 08 g, 1967; British patent application No. 1,345,567 Cl. C 08 g 20.18, 1974; U.S. Pat. No. 3,799,899 Cl. 260-78, 1978; U.S.S.R. Inventor's Certificate No. 614,120 Cl. C 08 G 69/16, Bulletin of Inventions Industrial Designs and Trademarks, No.25, 1978; British Application No. 1,402,567 Cl. C 08 68/18, 1975).
It is known that in the synthesis of polyamides, in particular, polycaproamide (polyamide-6) great attention is paid to filtration of the monomer melt to make it free from contamination (G. Klare, E. Fritzsche, F. Groebe "Synthetic Polyamide Fibres", "Mir" Publishers, Moscow, 1966, p. 101; G. Kudryaevtsev, M. Nosov, A. Volokhina, "Polyamide Fibres," Khimija Publishing House, Moscow, 1976, p.81). Various filter apparatus and filter fabrics are recommended for this purpose (cf. K. Fishman, N. Khruzin, "Manufacture of Kapron Fibre," Moscow, "Khimiya" Publishers, 1976, p. 48; L. Volf, B. Haitin, "Manufacture of Polycaproamide," Moscow, "Khimiya" Publishers, 1977, p. 68 and 81).
In the literature there are examples illustrating the use of disintegrated polyamide fibers as a filtering aid for a built-up filtering bed (cf. U.S. Pat. No. 3,880,754 Cl. 210-75, 1975; FRG Pat. No. 2,111,855 Cl. B01 D 39/04, 1976). However, these materials which are suitable for filtering a melt of caprolactam (at a temperature of from 100.degree. to 120.degree. C.) turn to be inapplicable for the filtration of a melt of .omega.-dodecalactam due to the necessity of applying a higher temperature. The use of filtering materials at a temperature within the range of from 200.degree. to 250.degree. C. has been disclosed in the literature.
At such temperatures, use can be made of, for example, a zeolite filtering partition (cf. Japanese Pat. No. 49-43198, Cl. 72 C 340.1, 1974). However, the procedure for partition manufacture is very complicated and necessitates the use of rather cumbersome equipment for its manufacture and use. The same disadvantages are inherent in metal-ceramic filters (cf. British patent application No. 1,313,795 Cl.B 01 D 39/10, 1973; U.S. Pat. No. 3,746,642; Cl. 210-46, 1974; French Pat. No. 2,252,865 Cl. B 01 D 39/06, 1975).and filtering blocks made of metal screens (cf. U.S. Pat. No. 3,679,057 Cl. 210-223, 1972). Furthermore, for the manufacture of metal-ceramic filters there are consumed up to 30-40% of alloying dopes, thus causing an essential increase in the production costs of the filtering materials. A method is known in the art for producing polyamide which comprises the following operations.
A mixture of the starting reagents, i.e.: dodecalactam melt, water, a catalyst and a molecular-mass regulator are supplied into the reactor by means of a metering device; in the reactor the first stage of polymerization is conducted at a temperature within the range of from 270.degree. to 320.degree. C. under a pressure of from 60 to 120 atm. Then the partly reacted mixture is fed into a throttling device, wherein evaporation of water occurs and the water is withdrawn from the process as steam.
Then the melt is throttled to a lower pressure and fed into the second reactor to the stage of polymerization, wherein a further build-up of the molecular mass occurs and the residual water is distilled-off under vacuum. The final polymer is discharged from the second reactor (cf. U.S.S.R. Inventor's Certificate No. 614,120 Cl. C 08 G 69/16).
This process features the disadvantage residing in that upon melting and accumulation of the melt of the starting .omega.-dodecalactam prior to metering thereof into the reactor, contact inevitably takes place between .omega.-dodecalactam and oxygen, despite all the measures taken to avoid it. Thus, there is used sealed equipment, continuous purging of the system and keeping an overatmospheric pressure therein by means of nitrogen containing less than 0.001% of oxygen. As a result of the contact with the residual oxygen at a temperature within the range of from 180.degree. to 250.degree. C. a certain portion of .omega.-dodecalactam is oxidized to give a substance which, while taking part in the polymerization, contaminates the final product; it is present in the form of minor inclusions of black colour which impair physico-chemical properties of the material.
Thus, prior to polymerization the amount of impurities in the melt of .omega.-dodecalactam is as high as 103 g/ton of the monomer.
To purify the polymer from contaminants, it should be subjected to manual sorting thus causing increased losses of the product and additional labour consumption.